The present invention relates to interlaced raster-scan television displays in general and, more particularly, to a method and apparatus for improving the display of objects, such as, computer-graphics on an interlaced raster-scan television.
The invention is primarily intended for use with high-resolution television (on the order of 1,000 scan lines), where some of the visual display problems are more pronounced, but is also applicable to other resolutions. In the discussion below, a 1,024-line horizontal-scan display will be used for purposes for explanation and illustration and a knowledge of television operation will be assumed.
One of the major problems with an interlaced display is frame-rate flicker of horizontal (scan line direction) lines. With computer-generated output, a horizontal line is likely to appear on a single scan line. This will then flicker at the frame refresh rate of the television. Especially, if there are a number of horizontal lines in the same field, this flicker is very noticeable and objectionable to an operator, and can cause headaches, eyestrain, etc. One approach to the problem has been the use of long-persistence phosphors. These help, but flicker is still noticeable, and not all the color television phosphors are available with long persistence (e.g. blue). Another approach is to simply decrease display brightness, where the eye is less sensitive to flicker. However, this approach makes the display hard to see in a normally illuminated room. A third approach is to increase the frame refresh rate above the normal 25 to 30 Hz. To be effective, this requires a considerable increase in the various system bandwidths.
One problem is the bandwidth required to display a picture. A reasonable display will have pixels that are, or are nearly, square (e.g. 1,280 by 1,024 in the example). For a small pixel size, high bandwidth is required in the video generation, transmission, reception, and the final CRT itself. To go from an existing to a higher resolution, normally requires an increase in all these bandwidths. Associated with this technique may be higher cost and complexity, reduced transmission distance and dimmer CRT picture (to get a smaller spot size).
One problem with computer-generated displays is the stair-stepping ("jaggies") that occurs with off-axis lines. This is due to the discrete nature of the data, where a line appears at one pixel, or the next, but not halfway between. Jaggies usually do not affect system usefulness, but are noticeable and aesthetically unpleasing. There are a number of "gray scale dejaggy" algorithms that can make such lines appear quite smooth. However, there is a computational overhead (giving a longer time to display an image) and the lines get fatter and slightly blurred (giving less resolution).
It is, accordingly, a general object of the invention to provide a method and apparatus for improving the appearance of objects displayed on an interlaced raster-scan television.
It is a specific object of the invention to reduce frame-rate flicker of scan lines in an interlaced raster scan display.
It is still another object of the invention to reduce the apparent stair-stepping that occurs with off-axis lines in an interlaced raster-scan display.
It is a feature of the invention that positionability of graphics elements is maintained while improving the display of such elements on an interlaced raster-scan display.